This invention relates to air/fuel ratio control systems for internal combustion engines, and more particularly to a device provided in such a control system for interrupting, at low atmospheric pressure, operation of a fail safe device for an O.sub.2 sensor for detecting the concentration of oxygen in the engine exhaust gases.
An air/fuel ratio feedback control system for internal combustion engines has already been proposed by the applicants of the present application e.g. in U.S. Ser. No. 281,118 filed July 7, 1981, now Pat. No. 4,380,985, which comprises an O.sub.2 sensor for detecting the concentration of oxygen present in the exhaust gases emitted from an internal combustion engine, and air/fuel ratio control valve having a valve body disposed to determine the air/fuel ratio of an air/fuel mixture being supplied to the engine, and an actuator arranged to drive the air/fuel ratio valve in response to an output signal of the O.sub.2 sensor, thus to carry out feedback conrol of the air/fuel ratio responsive to changes in the above oxygen concentration so as to keep the air/fuel ratio at a predetermined value.
The O.sub.2 sensor used in the above air/fuel ratio feedback control system is comprised of a sensor element made of stabilized zirconium oxide or a like material. The O.sub.2 sensor is adapted to detect the concentration of oxygen in the engine exhaust gases in such a manner that the output voltage of the O.sub.2 sensor varies correspondingly to a change in the conduction rate of oxygen ions through the interior of the zirconium oxide or a like material, which corresponds to a change in the difference between the oxygen partial pressure of the air and the equalibrium partial pressure of the oxygen in the engine exhaust gases.
The internal resistance of the O.sub.2 sensor which determines the output voltage of the O.sub.2 sensor also varies with a change in the degree of activation of the sensor. Thus, the activation of the O.sub.2 sensor can be determined by measuring the internal resistance of the sensor. When inactive, the O.sub.2 sensor has its output voltage variable within a small range and unable to vary in quick response to changes in the concentration of oxygen in the engine exhaust gases. Therefore, the air/fuel ratio feedback control operation is not initiated until after the O.sub.2 sensor has become fully activated. During the feedback control operation which is thus initiated after full activation of the O.sub.2 sensor, the air/fuel ratio of the mixture is controlled to values appropriate for the operating condition of the engine (which is a function of engine rpm, engine load, etc.) by means of the aforementioned air/fuel ratio control valve which is driven by an actuator such as a pulse motor in response to changes in the output voltage of the O.sub.2 sensor.
Therefore, it goes without saying that a failure in the O.sub.2 sensor would make it impossible to properly carry out the air/fuel ratio control operation. If in the event of O.sub.2 sensor failure the air/fuel ratio feedback control operation is continued without taking any emergency measures, the air/fuel ratio might be controlled to abnormal values, adversely affecting the driveability and exhaust emission characteristics of the engine. Thus, in order to always ensure proper air/fuel ratio feedback control, measures are indispensable for immediately detecting a failure in the O.sub.2 sensor and its related parts and taking appropriate actions upon detection of such failure.
Means for detecting a failure in the O.sub.2 sensor have also been proposed by the present applicants, which include a type adapted to detect whether no inversion occures in the output level of the O.sub.2 sensor over a predetermined period of time during the air/fuel feedback control when the O.sub.2 sensor is activated, as proposed in U.S. Ser. No. 299,382 filed Sept. 4, 1981, and a type adapted to detect whether the O.sub.2 sensor becomes activated within a predetermined period of time after the engine cooling water temperature has risen above a predetermined value during the air/fuel feedback control, as proposed in U.S. Ser. No. 299,675 filed Sept. 8, 1981. These proposed failure detecting means are both adapted to control a fuel metering device so as to achieve a predetermined air/fuel ratio compensated for atmospheric pressure, upon detection of a failure in the O.sub.2 sensor.
On the other hand, in controlling the air/fuel ratio by the use of an ordinary fuel supply system, the mixture being supplied to the engine becomes excessively rich during engine operation at a high altitude where low atmospheric pressure prevails. To avoid this, according to the aforementioned air/fuel ratio feedback control system proposed by the applicants, the feedback control is effected such that the actuator is moved in response to the output signal of the O.sub.2 signal in the direction of leaning the mixture so as to keep the air/fuel ratio at a theoretical value. However, even with this feedback air/fuel ratio correction, the ambient atmospheric pressure drops so largely that the mixture remains too rich in the event that the excessively rich mixture has an air/fuel ratio falling outside a limit value within which the feedback air/fuel ratio correction is possible. If the engine operation is continued in such condition, the output level of the O.sub.2 sensor remains high above a predetermined reference level, that is, no inversion occurs in the output level of the O.sub.2 sensor over a predetermined period of time. Also, when the engine is started under low atmospheric pressure at a high altitude, sometimes the output voltage of the O.sub.2 sensor does not drop below a predetermined reference voltage provided as a criterion for activation of the O.sub.2 sensor, even after a predetermined period of time has passed from the start of the engine. In these events, the O.sub.2 sensor--fail safe device undesirably operates to carry out fail safe functions such as warning and diagnosis, though the O.sub.2 sensor and its related parts are then not out of order.